This invention has to do with a thermocouple for use in applications where it is essential to measure a rapidly changing temperature of a material, particularly a liquid, with a minimum of time delay. For instance, when the temperature of the liquid being measured makes a step change, say, 50 degrees Fahrenheit (F), a fast acting thermocouple as envisaged by this invention achieves the same change in less than 2 seconds. Faster acting thermocouples, with the ability to make this temperature step change in a few tenths of a second are commercially available today, but they do not have other characteristics that are included in the present invention that are required for the applications for which the present invention is intended to be used.
There are many different types and shapes of devices for measuring the temperature of materials, both solid and liquid, which are commercially available today. The devices of primary interest here are those that can measure the temperature at a given location, and at the same time, operate a switch indicating that the temperature has reached and/or exceeded a pre-set set point temperature. Among these are (a) Bourdon tube type consisting of a bulb, tube, expansion device with internal liquid, and switch, (b) direct acting temperature switch, (c) thermocouple sensor (TC) and a readout instrument with switch, and (d) resistance temperature device (RTD) and readout instrument with switch.
The applications of interest here are those in which there is a liquid, such as a process liquid, wherein the temperature may vary, and vary quite substantially, and the pressure at the point where the temperature is measured may be high and may be cyclic at a high frequency. For example, the liquid may be an oil, such as an automatic transmission fluid (ATF). The process may be a fluid drive wherein the oil is used to transmit power from an impeller to a runner, wherein the impeller is a vaned structure fixedly attached to an input shaft that is driven by a prime mover such as a Diesel engine, and the runner is a vaned structure fixedly attached to an output shaft, and the output shaft is connected by a power train to, and functionally transmitting power to a load, such as a comminuting mill, for example, a hammer mill.
Extensive discussion of fluid drive technology is provided in U.S. Pat. Nos. 5,331,811, 5,315,825, and 5,303,801. A detailed discussion of the particular application for which this thermocouple was first devised is provided in a co-pending application, identified as U.S. Ser. No. 10/171,125. In many fluid drives in power plants, normal mineral oil such as light turbine oil is used, and the normal operating temperature range may be between 130 degrees Fahrenheit and 180 degrees Fahrenheit, being limited to 180 degrees F. in order to maintain a long life without deterioration, or oxidation, of the oil. In other applications using ATF oil, the upper limit of the temperature range may be 220 degrees Fahrenheit, with oil changes from time to time. In other applications, certain synthetic oils, such as Mobil 627, the normal operating temperature range may extend to 250 degrees Fahrenheit.
There are many temperature measuring devices that are commercially available that will measure temperatures in these ranges. However, when fluid drives experience fault conditions that stop the output shaft, that is, rotation of the output shaft and the attached runner ceases, it is very likely that the oil within the fluid drive impeller and runner chamber as well as the impeller, runner, and impeller casing will experience much higher temperatures, on the order of 300 degrees F., sometimes up to 450 degrees F., and the rise in temperature will be experienced extremely rapidly after the initiation of such a fault. Almost always, a fault of this type occurs in the equipment driven by the fluid drive, examples being (1) a pump for which the supply of the liquid being pumped stops, and the pump overheats and freezes, so that the pump shaft can not rotate, and any shafting connected to it including a fluid drive can not rotate; and (2) a hammer mill that becomes jammed, and can no longer rotate.
Such high temperatures, particularly when they occur rapidly, can damage the components of the fluid drive rotating elements, bearings, surrounding housings, the oil, the oil conditioning system, oil pumps, and seals. When the paint on the fluid drive, oil pipes, heat exchanger, and pumps is discolored and peels off, almost always, severe damage has occurred to the internal components. The heating occurs when the fluid drive impeller and runner cavities are filled, or partially filled, with oil and the impeller continues to rotate while the runner stops, causing power to be dissipated in the oil within the fluid drive element which is many times the power loss dissipated in the oil during normal operation, with the obvious consequence that the oil temperature rapidly soars to the aforementioned excessive temperatures.
Clearly, it is advantageous to avoid or to minimize the temperature excursions of the oil and other parts of the fluid drive system.
After such an event occurs, the damage assessment is made and will often not address the instrumentation, including instrumentation of all types. The instrumentation may be severely damaged by the event and the damage may not be detected because such events are rare and there is little experience with the damage such over-heating events can do. Every piece of instrumentation must be thoroughly inspected and tested. Of most concern in this application is temperature measuring instrumentation. One example of such damage is this: A Bourdon tube based device can be damaged by an over-temperature event because the fluid expands more than is anticipated and will cause some portion of the bulb or measuring capsule to expand and yield, causing the instrument to have an offset in its measuring and switching capability.
Another example occurs when the sensor bulb is properly located, but the instrument with switch is mounted onto the side of a fluid drive subject to process temperature, instead of being mounted as anticipated on a remote stand not affected by the temperature of the fluid drive. A problem occurs when the instrument is calibrated in a test room at 70 degrees F., and the instrument is mounted on a fluid drive the temperature of which varies substantially. It has been found that the pre-set temperature at which the switch changes state in the test room bears little relation to the temperature at which the switch actually operates when it is mounted on a fluid drive.
To avoid this type of problem, a sensor and an instrument must be selected and the instrument must be located so that the process temperature is measured with sufficient accuracy and repeatability and so that the switch operates at the expected pre-set temperature with sufficient accuracy and repeatability.
Experience has shown that there is no appreciable time delay in most instruments and switching devices for thermocouple sensors. Almost all of the time delay is in the thermocouple and this delay is related to the mass of metal to be heated (or cooled) and any insulation between the liquid outside of the external profile of the thermocouple and the junction of the two metals in the bead. It is exceedingly important that the temperature excursion be detected as rapidly as possible after the fault occurs that causes the over-temperature excursion in order to minimize the maximum temperature experienced and in order to minimize the duration of such over-temperature event.
Self contained sensor and switch units, such as produced under the name of Kaiser have been found to be damaged and non-functional after over-temperature events with temperatures over 450 degrees F., confirming prior experience that instruments containing switches that were mounted on the side of fluid drives did not work properly when exposed to high temperature conditions. Consequently, one way to be certain that the sensor and mated instrument with switch will operate with sufficient accuracy, and repeatability, for millions of over-temperature cycles, is this: The sensor must be located in the process and the sensor must be rated for temperatures that are well above the anticipated maximum temperature to be experienced by the sensor in an over-temperature event, and the instrument with switch must be mounted remotely in an environment where the temperature and vibration do not affect the operability of the instrument readout and switching function. This is achievable using presently available thermocouple technology and available instrumentation with readouts and switches, so long as fast acting response of indicated temperature at the readout instrument and switch is not required.
Three well known American vendors of a wide range of thermocouples and the associated instrumentation, as well as many other types of temperature, pressure and other sensing and readout devices are Pyco of Pennsylvania, Minco of Minnesota, and Omega of Connecticut. There are others, one of which is FW Murphy of Oklahoma, that offer instrumentation for mobile equipment, typically using 12 to 24 volt DC power for the power supply to operate the instruments.
When measuring temperature of liquids, including oil, in pipes or tanks, it is extremely common to use a thermowell for the thermocouple, RTD, bulb, or other sensing device. A thermowell is essentially a small diameter pipe that is sealed at the end that extends into the liquid or flowstream and is threaded or welded into the pipe or tank. The use of a thermowell permits the sensing device to be removed and replaced without shutting down or otherwise endangering the process. For the present objective of having a fast acting thermocouple, the problem of using a thermowell is that the thermowell has a substantial heat capacity, and the thermowell must be heated to the temperature of the liquid before the thermocouple inside can be exposed to the temperature of the thermowell, which means that a separate thermowell can not be used for a fast acting thermocouple as intended by the present invention.
A thermocouple is made by welding the ends of a pair of wires into a bead or junction, and using a voltmeter capable of reading millivolts to measure the voltage at the other end of the pair of wires. As the temperature of the junction changes, the voltage induced into the wires changes, a very simple and reliable method for measuring temperature at a point remote from the readout instrument. The smaller the bead is that forms the junction, the faster the response time. The critical design problems for a thermocouple for a specific application relate to such items as the materials from which to make the wires, wire diameter, insulation of the wires, insulation for the junction, overall size, shape, length and diameter, as well as protective coating or outside shield to address survivability and endurance when the thermocouple is subjected to mechanical vibration, liquid absolute pressure, liquid jetting pressures, over-temperature excursions, and the like.
Typical materials used to make the pair of wires, hence, the thermocouple junction, are Copper-Constantan, Iron-Constantan, and Chromel-Alumel. In other words, one wire is copper from the junction to the readout device, and the other wire is constantan from the junction to the readout device. There are other thermocouple junctions (see, for example, U.S. Pat. Nos. 3,942,242 and 4,224,461). Each combination of materials of a junction has a useful temperature range, and has a unique millivoltage vs. temperature scale. There is overlap in the useful temperature range, so it is critical to know the materials of the junction in order to be able to convert the millivolt reading to temperature. The length of the wires to the readout device is not critical, up to several hundred feet can be used. It is critical that the material of each wire from junction to readout must be the same, that is, one wire is entirely copper of the same constituency throughout, and one wire is entirely constantan of the same constituency throughout, because the temperature of intermediate junctions of non-identical materials will affect the voltage at the readout device.
An exposed thermocouple junction at the end of a pair of wires, with the wires typically covered by insulation such as woven fiberglass, can be inserted directly into a liquid, and this type of thermocouple will provide very accurate readings and will be very fast acting. However, in most applications, it is essential that there are no leaks of the process liquid through the wall of the pipe or vessel at the location where the thermocouple is inserted, which means that some form of shield or protective coating, such as a tube with the end welded closed, be used, along with a fitting mounted in the pipe or tank to seal the connection to the tube. It is very common to use a piece of stainless steel tubing, ⅛ inch, 3/16 inch, or ¼ inch diameter, as an outer shield, or sheath, and to insert the thermocouple junction and wires into the tube, or outer shield, stopping when the thermocouple junction hits the closed end of the tube. With regard to grounding of the thermocouple, two choices exist: (a) either the thermocouple junction is permitted to hit the closed end of the tube and remain in contact with the tube, called a grounded thermocouple, or alternately, (b) the thermocouple junction is coated with an insulating material and inserted into the tube stopping when the insulation hits the closed end of the tube, this arrangement being called variously and interchangably an ungrounded thermocouple, an insulated thermocouple, or an isolated thermocouple. A variation of the grounded thermocouple is to use a tube with both ends open, and to insert the junction through the tube until it begins to exit from the other open end, and to weld the junction to the end of the tube, so that the tube is sealed and the junction can not be removed from the tip of the tube. A variation of both types, gounded and ungrounded, is that two pairs of wires, each pair of wires having a junction, can be inserted and used, called a duplex thermocouple. The advantage of a duplex thermocouple is that two separate temperature measurements of the same physical location can be measured and used to drive two different readouts, or simply, one thermocouple can be used, and if for some reason it fails, or is suspected of failure, which is more probable, the other thermocouple can be connected to replace the first.
With a tube fitting threaded or welded into the pipe or tank, the thermocouple assembly with protective outer sheath is inserted through the tube fitting with a grommet or metallic sealing ring and the outer locking cover of the fitting is tightened onto the sheath until a seal is made and the thermocouple assembly with sheath can not be removed.
The layer of insulation used in ungrounded thermocouples as described just above provides a delay in heat transfer from the outer protective sheath or tube to the thermocouple junction, hence, ungrounded thermocouples so designed do not qualify as fast acting thermocouples.
Minco makes a grounded thermocouple designed to be fast acting, called a Quick Tip, of which there are several versions. Others manufacture similar devices under other trade names. A common version is a small tube 0.093 inches diameter and perhaps 5/16 inch long, with the wires inserted through an open end of a tube until the wires begin to protrude from the second end of the tube and welded to the second end, sealing it. The total mass of this tube and junction is on the order of a few grams, and it heats very rapidly, with the indicated temperature by the readout device following behind actual changes of the temperature of the liquid by a few hundredths of a second. Another variation is a larger tube, perhaps, 3/16 inch diameter with a flat end; the end is a flat round disk that extends beyond the outside of the tube to a diameter of perhaps ¼ inch, providing a larger area to collect heat, such as would be useful in a steel backed, Babbitt lined bearing. In such an application, a hole the size of the OD of the tube is drilled through the Babbitt and into the steel behind the Babbitt layer sufficiently deep to install the thermocouple, Babbitt is removed around the hole sufficient for the flange to seat on the steel backing, and another hole is drilled from the outside of the bearing communicating with the hole where the thermocouple tube is to be inserted. After the wires are inserted into the hole from the Babbitt side and extending through the hole and beyond the outside of the bearing and the thermocouple is inserted and the flange is seated, the flat disc, or flange, at the end of the thermocouple is tinned and Babbitt is added. After it cools, the Babbitt surface is machined to suit the surrounding shape of the Bearing surface.
It is essential that the thermocouple is not over-heated and damaged by the high temperatures used to tin the end and to get the Babbitt melted and installed. A version of Quick Tip thermocouple comes with the flat disc at the end already tinned to reduce the time that it is exposed to elevated temperatures, and to assure greater bonding of the Babbitt layer to the tip of the thermocouple. In almost all applications, this arrangement works as intended. However, occasionally, the pressures of the lubricating oil film on the Babbitt layer, and hence, on the flat disc at the end of the thermocouple cause the flat disc to dimple inwardly. Sometimes, the pressures are so great that the end of the thermocouple is pushed into the hole and the sensor is destroyed.
A problem that often occurs with grounded instrumentation, including thermocouples, sometimes unknown to the designer or user, is that other electrical equipment in the area can induce stray voltages into the instrumentation, such stray voltages being either direct current and/or alternating current. Occasionally these stray currents are traced to such conditions as improper grounds or loose ground wires. Since the currents in most instrumentation are very small, on the order of milliamps, such stray voltages induced by the ground loops can easily be sufficiently large to affect, to confuse, or in extreme cases, to mask, actual readings. It is for this reason that almost all instrumentation is ungrounded, unless it is necessary to use a grounded design to get the desired information, such as the Quick Tip thermocouple for a Babbitt bearing, described above.
The thermocouple of this invention is designed and manufactured as a fast acting, ungrounded, high temperature thermocouple that can resist pressures to a few hundred pounds per square inch (psi), perhaps higher pressure in other versions, can survive in a vibrating environment such as fluid couplings attached to and driven by Diesel engines, and fluid drives driving boiler feed pumps or fans in power plants, and can be used with any readout device and switching device suitably designed and calibrated for the materials used in the junctions and wires.